Extend BinderChannel to support cross-user communication and introduce BinderChannelCredentials (io.grpc.binder.*) in experimental.

BinderChannel, as specified in [gRFC L73](https://github.com/grpc/proposal/blob/master/L73-java-binderchannel.md) only supports the cross-process communication within the same Android user. However, some special Android applications need to do cross-user communication either using [Context.bindServiceAsUser](https://developer.android.com/reference/android/content/Context#bindServiceAsUser(android.content.Intent,%20android.content.ServiceConnection,%20int,%20android.os.UserHandle)) or [DevicePolicyManager.bindDeviceAdminServiceAsUser](https://developer.android.com/reference/android/app/admin/DevicePolicyManager#bindDeviceAdminServiceAsUser(android.content.ComponentName,%20android.content.Intent,%20android.content.ServiceConnection,%20int,%20android.os.UserHandle)).

TargetUserHandle is introduced in BinderChannel to enhance this use case. In the meanwhile BinderChannelCredentials is introduced in experimental stage to capture arbitrary arguments needed by DevicePolicyManager or other Android components to establish a binder connection.

Comienza leyendo gRFC L73 y las APIs de Android bindServiceAsUser y bindDeviceAdminServiceAsUser. Luego localiza BinderChannel y el paquete experimental io.grpc.binder; el trabajo estará completo cuando los destinos entre usuarios y BinderChannelCredentials admitan los argumentos arbitrarios necesarios para establecer esas conexiones de Binder.

Escrito por el modelo de indexación a partir del texto del issue.